www.linmodems.org should give you a pointer as to which internal modems might work. Best approach, however, is to use an external serial(RS-232) modem. You should be able to pick up a second-hand one for £10 or a new one for about £20-£40.

Real internal (PCI) modems that need no special driver can be found if you look hard enough, though they tend to be much more expensive than softmodems. For years I used one made by Multitech. It cost about £100, but worked a treat.